The invention relates to an open-end spinning machine with a plurality of spinning units, each containing a spinning rotor connected to a vacuum source. An air current is generated by the vacuum source to feed fibers fed by a feeding means and opened by an opening device to this spinning rotor. The invention further relates to apparatus for conducting a piecing step comprising devices for returning a thread end into the spinning rotor for piecing onto a fiber ring deposited therein and for taking off the thus-pieced thread, this apparatus being equipped with additional devices for affecting the rotor speed and the quantity of fed fibers during the piecing step.
As an aid to understanding the present invention, the following prior patents are incorporated herein by reference thereto: U.S. Pat. No. 3,892,062 (reissue application Ser. No. 812,411 filed July 1, 1977, now U.S. Pat. No. Re. 30,167); U.S. Pat. Nos. 3,924,393; 3,942,311; 3,950,926; 3,962,855; 3,987,610; and 4,059,946.
In an open-end spinning machine of the aforementioned type, it is known from U.S. Pat. No. 3,987,610 (corresponds to DOS [German Unexamined Published Application] No. 2,360,296) to effect the actual piecing operation, i.e. the return of the thread end, the attachment thereof to the fiber ring, and the taking off of the thus-pieced thread, within a time period wherein the previously braked spinning rotor is again returning to its operating speed. To make a piecing step possible at all, a fiber ring must have been deposited in the spinning rotor, to which the thread end can be pieced. To obtain this fiber ring, a certain amount of fibers must be fed into the spinning rotor. Within the spinning rotor, the fibers are retained in the zone of the fiber-collecting groove by centrifugal forces acting on the fibers. If the centrifugal forces are missing, i.e. when the rotor is at a standstill, or if the centrifugal forces are too low, i.e. when the spinning rotor runs at low speed, the fibers are entrained by the escaping conveying air from the rotor and removed by suction. To form the fiber ring from a specific quantity of fibers, feeding must therefore begin at a certain minimum speed; i.e. at a speed taking care of providing sufficient centrifugal force.
Under practical conditions, it was found that, in spite of maintaining all conditions most carefully, differing pieced thread sections are produced time and again, exhibiting deviations from other pieced thread sections with regard to tear strength and shape.
The invention is based on the problem of providing a possibility for refining the piecing operation so that uniform pieced thread sections are obtained with an even greater degree of certainty. The invention contemplates solving the mentioned problems by the provision of devices serving for affecting the air current, which latter effects the fiber transport from the opening device to the spinning rotor, during the piecing operation.
The invention starts with the realization that the speed of the air current and consequently also the velocity of the conveyed fibers depends practically exclusively on the vacuum [subatmospheric pressure] ambient within the spinning rotor. This feeding speed must be adapted, during normal operation, to the speed of the rotor so that the wall, upon which the fibers impinge, has a markedly higher velocity than the arriving fibers, so that the fibers, by entrainment along the wall of the spinning rotor, are drawn from the fiber feed duct and are thereby stretched and are deposited, in this stretched condition, on the wall of the rotor. These necessary velocity relationships, however, are not present in case the piecing operation is conducted at a reduced rotor speed. In this case, it happens that the fibers impinge with excessive speed on the rotor wall and are not drawn off therefrom but rather are crushed against the wall. In such case, they are then deposited in a tangled, irregular position, forming so-called "bird's nests". The invention provides that, during the piecing step, the air current can also be affected so that the transport velocities of the fibers can be adapted to the conditions existing during the piecing step.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration, several embodiments in accordance with the present invention.